Superhydrophobic surfaces are those that display a water contact angle larger than about 150°. Superhydrophobicity and self-cleaning are properties of a Lotus Effect surface. A Lotus Effect surface arises when the surface is covered with a low surface free energy material, which provides a relatively high contact angle with water, and has a very fine rough structure.
Surfaces with a fine rough structure allow air to be trapped in the fine structures to reduce the contact area between the liquid and the surface. For example, when a water drop is placed on a lotus plant surface, air is entrapped in the nano-rough surface structures and only the tip of the nanostructures contact the water drop. The water contact area is only 2-3% of a droplet-covered surface of a lotus plant leaf. Therefore, the water gains very little energy through adsorption to compensate for any enlargement of its surface and the water forms a spherical droplet with the contact angle of the droplet depending almost entirely on the surface tension of the water.
The relationship between the surface water contact angle and the surface structural geometry, the Wenzel roughness, is given by the Cassie equation:cos θA=rf1 cos θY+f1−1  Equation 1where the r is the ratio of the actual solid-liquid contact area to its vertical projected area (Wenzel roughness factor), θA is the apparent contact angle on the rough surface, and θY is the contact angle on a flat surface as per Young's equation, f1 is the solid surface fraction. This roughness to form a Lotus Effect surface can be produced by etching a nanoscale rough structure on a hydrophobic surface; coating a thin hydrophobic film on nanoscale rough surface; or simultaneously creating a rough structure with a decreased material surface energy.
Superhydrophobic properties are desirable for many applications. A durable superhydrophobic and self-cleaning coating would be invaluable for use in: high voltage industry to limit leakage currents and to prevent flashover; microelectromechanical systems (MEMS) industry to limit or prevent stiction; and anticorrosion of metal coatings. Other applications for superhydrophobic surfaces include: directed liquid flow in microfluidics; antifouling in biomedical applications; and transparent coatings in photovoltaics devices.
Superhydrophobic surface coatings of architectural glass, smart phones, touch screens, and many other articles that need to stay smudge free and clean and water free would benefit if the coating is transparent and durable. Such coatings could be used to reduce or avoid fogging, for example on bathroom mirrors, shower doors and the interior of a car windshield. The coating would display a degree of ice-phobicity, which would keep turbine blades of windmills running in cold climates, and reduce ice build-up on ships, planes, cars, trucks, and architectural structures.
To this end, the development of transparent superhydrophobic coatings that are durable and can be applied at or near ambient temperatures and pressures is desirable.